The invention relates to a radio-frequency signal receiving unit particularly for Nuclear Magnetic Resonance imaging machines, comprising:
a) a signal receiving coil (1), whose outputs (101, 201) are connected to a signal amplifier (2) PA1 b) the coil (1) being in balanced connection with the signal amplifier (2), i.e. connected thereto in such a manner that none of its two outputs (101, 201) is permanently connected to the ground; PA1 c) passive balancing means (4), the amplifier (2) being interposed between the coil outputs and the said passive balancing means (4); PA1 d) the signal amplifier (2) comprising at least two active amplifying devices (3), each associated to an input to which one of the outputs (101, 201) of the coil is connected, PA1 e) active amplifying devices (3) having an output connected to a device for recombining useful signals and canceling out undesired signals, that is a device which is able to add signals in phase opposition and to cancel signals in phase from inputs.
In Nuclear Magnetic Resonance imaging machines and especially in "dedicated" machines, i.e. designed to detect images of individual parts of the body, the problem of effectively removing noise from the detected signal is particularly significant.
The mostly magnetic radio-frequency signal reemitted from the patient body is relatively feeble and combines with mostly electric noise components introduced by the body directly inside the detection volume: therefore, this noise must be canceled from the signal, which is also affected by thermal noise, so that is can be further amplified and that the data thereof can be correctly processed to reconstruct the image.
A well-known and effective solution to the problem of electromagnetic noise on the signals received by a receiving coil consists in using a Faraday cage, which is assembled in such a way as to enclose and contain the whole machine and the patient inside it, in order to provide effective shielding of the detected electromagnetic signals.
This choice certainly provides a solution to the problem of electromagnetic noise removal.
Nevertheless, the relatively big shape and size of a structure of this type make its construction particularly expensive and difficult, and anyway requiring plenty of time; the use of the machine would be more complex for the personnel and, further, if the machine is not properly integrated into an appropriate environment, it may induce a certain emotional impact on the patient and be psychologically unattractive.
A further solution consists in placing the patient body on the ground, while closing the apertures for entering and introducing the part of the body in the machine volume dedicated to Nuclear Magnetic Resonance imaging.
In this case, noise may be removed thanks to ground connection, since a specific path is provided for the noise signals picked up by the patient body: however, even this solution is not effective enough to satisfactorily remove the noise.
The electrical connection of the patient to the ground is relatively difficult due both to the anatomic structure and to the low conductivity of the human body.
In any case, the ideal ground connection conditions cannot be easily attained and a certain part of noise is equally transmitted inside the detection cavity.
Document U.S. Pat. No. 4,839,594 discloses a radio-frequency signal receiving unit showing the combination of features a) to e) listed above. The arrangement of U.S. Pat. No. 4,839,594 solves the problem of suppressing the electrically induced noise by the patient body. Nevertheless the amplifier introduces thermal noise, which amount is currently quantified by the so called Noise Figure. This thermal noise limits the overall signal to noise ratio that can be achieved by the system reducing the advantages aimed by the balanced coil configuration.